An ultrasonic diagnostic apparatus outputs driving signals from a transmission means so as to transmit ultrasonic waves from a probe on an object to be inspected, receives echo signals reflected by the object with the probe, and reconstructs an ultrasonogram (ultrasonic image) on the basis of the received signals. The probe consists of an array of oscillators, and the focal position of the ultrasonic waves in the object can be controlled by adding predetermined time lags to received signals of the oscillators upon receiving of ultrasonic waves. The method of shifting a focal position by changing the time lags is called dynamic focus.
It is desirable on an ultrasonic diagnostic apparatus that the waveform of the transmitted ultrasonic wave should be a pulse wave which is short in the direction of the time axis in order to improve the depth resolution, and have a large signal intensity in order to improve the SN ratio (Signal Noise Ratio). However, it is necessary to control the maximum intensity of the ultrasonic waves to such a level that they should not affect living bodies. Therefore, in order to increase transmission energy with suppressing the maximum intensity of ultrasonic waves, it is described in, for example, Japanese Patent Unexamined Publication (KOKAI) No. 2003-225237 and so forth to apply an encoded transmission technique widely used in the field of radars also to ultrasonic diagnostic apparatuses. In this technique, a single pulse waveform having a large peak intensity is diverged into a sequence along the time axis direction of signals each having a small peak intensity and transmitted to an object, signals reflected in the object are received, and then a decoding operation is performed with a demodulation filter to converge the signals along the time axis direction and thereby restore them into the pulse waveform having a large peak intensity.
As the code, the Barker code, Golay code and the like widely known in the field of radars can be used, and as the decoding filter, an autocorrelation filter, which performs autocorrelation operation, a mismatched filter, and so forth can be used.    Patent document 1: Japanese Patent Unexamined Publication (KOKAI) No. 2003-225237